1. Field of the Invention
This invention relates to a semiconductor device having a main thyristor and an auxiliary thyristor (which is also called a pilot thyristor) formed in a single semiconductor substrate, and more particularly to a sensitive thyristor having an improved noise-capability.
2. Description of the Related Art
In general, a thyristor is widely used to effect various control operations such as ON/OFF control of electric power. The basic structure of the thyristor is explained in the article by ADOLPH BLICHER in "Thyristor Physics" pp. 1 to 13. The thyristor in the OFF state is made conductive when a gate current is increased to a certain value with a preset voltage kept applied between the anode and cathode. The minimum gate current required for turning on the thyristor is called gate turn-on current I.sub.GT, and the sensitivity of the thyristor becomes higher when current I.sub.GT is set to be smaller. Gate turn-on current I.sub.GT is controlled by various methods so as to attain a desired ON characteristic.
When an abruptly rising noise voltage is applied between the anode and cathode of the thyristor in the OFF state, the thyristor may be turned on (erroneously triggered) even if no gate current is supplied. The turn-on operation is considered to occur as explained below. First, a junction capacitor between the P-type base region and N-type base region of the thyristor is charged by application of the noise voltage. As a result, a displacement current "Cdv/dt" (C is a capacitance of the junction capacitor) will flow over the entire surface of the junction area. The displacement current has substantially the same effect as the gate current, and at this time, the thyristor is turned on. In this case, voltage variation rate dv/dt immediately before the thyristor is turned on is called the critical off voltage rising rate. When the critical off voltage rising rate is larger, the possibility of the thyristor to be turned on by the noise voltage becomes smaller, and therefore it is preferable to set the critical off voltage rising rate to be larger. The above phenomena are generally called dv/dt characteristics.
The dv/dt characteristics and sensitivity of the thyristor are incompatible with each other. That is, when the sensitivity is set to be high, the dv/dt characteristic will be deteriorated, and when the dv/dt characteristic is enhanced, the sensitivity is lowered.
It is well known in the art that an amplifying gate type thyristor can be used to effect a method for controlling the ON and OFF conditions of a thyristor which is operated with a large driving ability (current capacity) by a small gate current. Such a thyristor is disclosed in "Thyristor Physics" pp. 120-121. The amplifying gate type thyristor includes a main thyristor having a large driving ability (requiring large gate turn-on current I.sub.GT) and an auxiliary thyristor having a small driving ability (which can be turned on by small gate turn-on current I.sub.GT). The main thyristor is constituted to have a cathode formed in the form of shorted emitter structure in order to enhance the dv/dt characteristic. In an equivalent circuit of the thyristor, the cathode of the auxiliary thyristor is connected to the gate of the main thyristor, and the anode thereof is connected to the anode of the main thyristor. When a gate current is supplied to the gate of the auxiliary thyristor and exceeds gate turn-on current I.sub.GT, the thyristor is turned on. As a result, current flowing from the anode to cathode of the auxiliary thyristor is supplied as a gate current to the gate of the main thyristor, thereby turning on the main thyristor. In this way, it is possible to turn on the main thyristor having the large driving ability (current capacity) with a gate current which is relatively small but is large enough to turn on the auxiliary thyristor.
As described above, in the amplifying gate type thyristor, the main thyristor having the large driving ability (current capacity) can be turned on by a relatively small gate current. However, the dv/dt characteristic is not sufficiently improved. This is because the main thyristor will be turned on when the highly sensitive auxiliary thyristor is turned on by a noise voltage even if the dv/dt characteristic of the main thyristor is sufficiently improved. In the amplifying gate type thyristor, the P-type base region of the main thyristor is electrically connected to the P-type base region of the auxiliary thyristor. Therefore, when a noise voltage is applied between the anode and cathode of the main thyristor, the highly sensitive auxiliary thyristor may be turned on even if the main thyristor having the sufficiently improved dv/dt characteristic is not turned on at this time. Thus, the dv/dt characteristic of the amplifying gate type thyristor largely depends on the dv/dt characteristic of the auxiliary thyristor.
In recent years, microcomputers were widely used in the household electric appliances, and it has been required to directly drive the thyristor by an output (small gate current) of an IC. Therefore, it is strongly demanded to develop sensitive thyristors. As described above, however, when the gate sensitivity of the thyristor is enhanced, the dv/dt characteristic thereof is deteriorated, that is, the noise-capability thereof becomes low, causing an erroneous operation due to the noise voltage.